In living beings, tissues suffer from wear throughout life but they have a self-renewal capacity. If this renewal did not exist, the life expectancy of living beings would be considerably reduced. Specifically, stem cells are the ones which have this tissue self-renewal capacity. The self-renewal capacity is shown by the capacity to produce more stem cells and, in addition, to generate cells of one or more differentiated cell types.
The replicative decrease of stem cells involves the degeneration and death of the different tissues, either in an acute manner (infarctions) or in a chronic manner (degeneration-aging).
Due to the mentioned self-renewal capacity, cell therapy with stem cells is currently applied, among others, in: heart pathologies, corneal pathologies and pelvic floor pathologies. Fistulas, especially in patients with Crohn's disease, are also treated.
Stem cells can be classified according to their differentiation potential as: totipotent, pluripotent and multipotent.
Totipotent stem cells can grow and form a complete organism, forming both embryonic components (such as the three germ layers, the germline and the tissues which will give rise to the yolk sac, for example) and extraembryonic components (such as the placenta). In other words, they can form all the cell types.
Multipotent stem cells are those which can only generate cells of their own germ layer or germline of origin, for example: since a bone marrow mesenchymal stem cell has a mesodermal nature, it will give rise to cells of that layer such as myocytes, adipocytes or osteocytes, among others.
Pluripotent stem cells cannot form a complete organism, but they can differentiate into cells from the three germ layers: (a) ectoderm, which is the origin of the nervous system, the respiratory system, upper digestive tract (stomodeum), the epidermis and its adnexa (hair and nails) and the mammary glands; (b) endoderm, which is the origin of the intestine, the liver, the pancreas, the lungs and most of the internal organs; and (c) mesoderm, which is the origin of the skeletal system, the muscles and the circulatory and reproductive systems. They can also form any other type of cell from the germ and the yolk sac.
The fact that pluripotent cells have the capacity to differentiate into such a large number of tissues makes them especially interesting for the design of new therapies in general and of regenerative therapies in particular. Currently, pluripotent stem cells in adult individuals are mainly obtained from bone marrow.
It is well known in the state of the art that when pluripotent stem cells are isolated and characterized from the extracted bone marrow sample, there are different well-established criteria in the state of the art which allow clearly classifying a stem cell as a pluripotent cell.
A first criterion would be to determine the capacity of the isolated cell to differentiate into tissues derived from the three germ layers (endoderm, ectoderm and mesoderm) using commercially available differentiation media (Prelle et al., “Establishment of pluripotent cell lines from vertebrate species, present status and future prospects”, Cell Tissues Organs, 1999, vol. 165, pp. 220-236).
A second criterion would consist of identifying the markers expressed by the isolated stem cell. For example, the expression of the gene OCT3/4 has been described as necessary for a stem cell to be pluripotent, Niwa et al. “Quantitative expression of Oct-3/4 defines differentiation, dedifferentiation or self-renewal of ES cells”, Nature, 2000 vol. 24, pp. 372-376. The expression of SSEA4 in pluripotent cells has been described by Venable et al. “Lectin binding profiles of SSEA-4 enriched, pluripotent human embryonic stem cell surfaces” BMC Dev Biol. 2005, vol. 21, pp. 5-15, and the expression of CD13 also in pluripotent cells is mentioned in the publication of Young et al., “Human pluripotent and progenitor cells display cell surface cluster differentiation markers cd10, cd13, cd56, and mhc class-I”, Proceedings of the Society for Experimental Biology and Medicine, 1999 vol. 221, no. 1, pp. 63-7.
Jiang et al. (cf. Jiang et al. “Pluripotency of mesenchymal stem cells derived from adult marrow”, Nature, 2002, vol. 418, pp. 41-47) have described the characterization of pluripotent stem cells in a cell population of the bone marrow. This population is the so-called MAPCs (multipotent adult progenitor cells). This article indicates that MAPCs express high levels of CD13, SSEA-4 and OCT3/4, typical pluripotent cell markers. Like embryonic stem cells, in MAPCs the activation of the transcription factors OCT-4, NANOG and REX-1 is detected, which factors are necessary for maintaining the cell in a proliferative and undifferentiated state. Cloning assays which demonstrate that it is a single cell which is capable of differentiating into tissues of any of the three germ layers (endoderm, mesoderm or ectoderm) are additionally included. The source of pluripotent stem cells described in this article is bone marrow. A general or local anesthesia is necessary in the extractions of stem cells from bone marrow. The method which is used to obtain them is aspirating the marrow content by means of the puncture of a bone. The material which is obtained passes through a series of screening processes to separate the cells. This makes obtaining these cells a complex and inefficient process.
Due to the foregoing, it is necessary to find alternative sources from which pluripotent stem cells can be extracted.